This invention relates to the art of polysulfide polymers, more particularly to thiol terminated liquid polysulfide polymers and hot melt applications, such as, hot melt applied sealants, extrusion of hoses and the like.
The use of thiol terminated liquid polysulfide polymer in caulks, sealants and the like for numerous applications is well known. Such materials have normally, until now, been chemically cured in situ.
When rapid cure was desired, a two part system was necessary requiring mixing with proper equipment just before use. One part chemical cure, on the other hand, was satisfactory but required that provision be made for time to reach structural integrity, in some uses requiring provision of storage facilities, support racks and the like. In production applications, the extra space and/or equipment added a cost factor which those skilled in the art would consider it desirable to eliminate.
Hot melt applied materials offer the potential to eliminate both the two part chemical cure requirement of in situ mixing and the lengthy holding requirement of one part chemical cure.
Although polysulfide rubbers are classified with other thermoplastic materials, the art has always considered that if a cured polysulfide polymer, or indeed most vulcanized rubbers, were to be heated to a point at which plastic flow was possible, irreversible degradation of the polymeric chain would follow and that upon cooling the resulting properties would be far poorer. In addition, a known degradation reaction of polysulfide polymeric chain occurs upon heating in the presence of acids.
That the incorporation of substantially anhydrous acetic acid in zinc oxide cured thiol terminated liquid polysulfide polymers results in cured rubbers capable of extrusion under heat and pressure with recovery of good properties on cooling is taught in the application of Eugene R. Bertozzi, Ser. No. 913,630, filed concurrently herewith. Screening of other carboxylic acids, both those structurally similar to acetic acid, and those more remote, has surprisingly revealed that of those tested, only levulinic acid is able to provide to a zinc oxide cured thiol terminated liquid polysulfide polymer the ability to be extruded in a fashion similar to the acetic acid containing cured polymers with recovery of good properties.
Up until now, the available hot melt sealants, such as butyl based materials, have not exhibited good structural properties on cooling and have required cumbersome mechanical retention systems or the application of a second curable sealant where some rigidity in the sealant is necessary as in glass sandwiches for insulated windows.
The present invention provides the ease of hot melt application with the structural support and other good properties obtainable from polysulfide polymer sealants.